In some applications such as telecommunications it is necessary to convert signals from analog form to digital form and vice versa. These conversions are performed respectively in an analog-to-digital converter (ADC) and a digital-to-analog converter (DAC). The data converters need fixed, temperature-insensitive reference voltages for best performance. These fixed reference voltages are commonly provided by a circuit known as a bandgap voltage reference circuit. The bandgap voltage reference circuit provides an output voltage which is stable over wide ranges of temperature by summing the difference in voltage across two bipolar transistors with different current densities with the voltage across a single bipolar transistor, and many such circuits are known in the art.
However, the data converters may require one or more reference voltages that are different than the bandgap reference voltage. For such a case, a differential amplifier can provide the reference voltage by utilizing a resistor divider network in a negative feedback configuration. The differential amplifier also provides the advantage of buffering the output of the bandgap voltage reference circuit to prevent it from being affected by changes in the load.
Deriving a voltage in this way involves a tradeoff between offset voltage and settling time. Any offset voltage in the differential amplifier changes the value of the reference voltage from the desired voltage. The settling time of the differential amplifier determines how quickly the reference voltage returns to its desired value after the load is changed. Ideally, the differential amplifier needs a very low offset voltage and a very fast settling time. One known approach is to use a switched-capacitor amplifier. The switched-capacitor amplifier makes possible low offset voltages, within the matching of the capacitors, through known offset compensation techniques. However, the switched-capacitor amplifier may load the output of the bandgap voltage reference circuit and cause a relatively-long settling time, for example when the bandgap voltage reference circuit uses an MOS process with parasitic vertical bipolar transistors. Furthermore, the switched-capacitor amplifier doesn't provide a continuous reference voltage at its output and is susceptible to noise which occurs during switching.
A second approach is to use a continuous-time amplifier. Known continuous-time amplifiers provide continuous reference voltages at their outputs, and do not load the bandgap voltage reference circuit, thus limiting settling time to the settling time of the amplifier itself. However they also suffer from higher offset voltages than switched-capacitor amplifiers which use the known compensation techniques referred to above. Known techniques of reducing the offset voltage of the continuous-time amplifier cause a corresponding increase in settling time. Thus what is needed is a continuous-time differential amplifier which provides both a low offset voltage and a fast settling time.